At 9:47 a.m., the truck loaded with books toppled over, trapping the driver and sending the back doors of the rig flying open, said CHP Officer Art Montiel.

The CHP issued a Sigalert and closed the ramp near San Francisco International Airport while firefighters worked to pull the trapped driver from the wrecked truck. Authorities said traffic was backing up in the area.

The driver's condition was not immediately known. The CHP was investigating the cause of the wreck.

The books were being manually unloaded, as the CHP said they would rip through the walls if the rig was uprighted.

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